1. Field of the Invention
The present invention is directed to a sheet forming system of a paper or cardboard machine, More specifically, the present invention is directed to a sheet forming system having a headbox with a nozzle and at least two guide mechanisms that guide screens in the region of the headbox nozzle over a guide surface.
2. Discussion of Background Information
A sheet forming system is disclosed in U.S. Pat. No. 4,358,342. This patent discloses the beginning of a sheet forming system of a papermaking machine with a twin wire former in the region of the headbox nozzle and the necked inlet gap of two screens. In the outlet region, the headbox nozzle has sectionally adjustable headbox lips that can influence or even out a base weight cross-direction profile of the paper web. Holding the upper and lower walls of the headbox nozzle in place requires a heavy and large support structure. To adjust the deformable upper or lower lip of the headbox, the relevant lip is supported on the heavily built base of die nozzle upper wall or nozzle lower wall.
Since this design requires a stable construction of the headbox up to the forward most region of the headbox nozzle, it is quite expensive. In addition, if the first two rolls of the screen section (that form the necked inlet gap) have large diameters, the headbox nozzle cannot be inserted far enough into the necked inlet gap. This results in a free jet that is too long, which decreases the quality of the manufacturing process.
German patent DE 41 05 215 C2 also discloses a sheet forming system in the region of the necked inlet gap of a twin screen section. In this system, the two rolls that form the necked inlet gap of the twin screen section are supported hydrostatically. The hydrostatic bearing across the width of the machine allows for compensation for deflection of the rolls. This uniformly maintains the inlet geometry of the necked gap over the entire width of the papermaking machine, even when small roll diameters are used.